Topic 5 Energy In Society

Question 1

What is tonnes of oil equivalent (TOE)?

Answer 1

This is the total energy released when a tonne (1000 kg) of crude oil is burned, which is taken to be 41.9 GJ.

Since the energy use of society is so large, often the mega tonne of oil equivalent is used (Mtoe).

Question 2

What is efficiency? (equation)

Answer 2

In a physical process, efficiency is defined as the amount of work output divided by the amount of energy supplied. It is therefore a dimensionless number often specified as a percentage.

Efficiency = (work output) / (energy supplied)

= W / Esup

Question 3

What is specific energy?

Answer 3

This is the total energy released from 1 kg of a fuel or power source.

Question 4

Steps for calculating the energy change in a reaction.

Answer 4

1 - write down all the bonds from the starting material that must be broken.
2- calculate the energy input required to break all the bonds identified in step one.
3- identify all the bonds that are formed on the right hand side.
4- calculate the energy released when all the right hand side bonds are formed.
5 - add the energy calculated from step 2 to the energy calculated in step 4. The sum is the energy absorbed or released by the reaction.

Question 5

What is an endothermic reaction?

Answer 5

This is any reaction where the change in energy is positive (the energy of the system increases).

Breaking bonds is endothermic.

Question 6

What is an exothermic reaction?

Answer 6

This is where the energy change is negative (the energy of the system decreases).

Making bonds is exothermic.

Question 7

What is ** bond enthalpy**?

Answer 7

This is the energy required to break a chemical bond or the amount of energy released when forming a chemical bond.

This is typically measured in kilojoules per mole.

Question 8

What in a mole?

Answer 8

This is an amount of a substance that contains a number of atoms or molecules equal to Avogadro’s constant (6.022 x 10^23).

Question 9

What is molar mass?

Answer 9

This is the mass of one mole of a substance. It is defined that one mole of carbon 12 has a mass of 12.0 g. In general one mole of a substance has a molar mass in grams that is equal to its relative atomic / molecular mass.

Question 10

What is the atomic mass unit?

Answer 10

The atomic mass unit (u) is one twelfth of the mass of a single atom of carbon 12, which is equal to 1.66 x 10^-27 kg.

The mass of any atom or molecule is equal to u times its relative atomic or molecular mass.

Question 11

What is nuclear fission?

Answer 11

The process whereby an unstable atomic nucleus splits into two roughly equal halves. There is a decrease in the total mass of the nuclei in a fission reaction, so this process results in the release of large amounts of energy according to Einsteins E = mc^2.

Question 12

What is EROI? (equation)

Answer 12

This stands for the energy return on investment and quantifies how much energy can be extracted from a fuel that costs one unit of energy to obtain.

EROI = (energy extracted) / (energy expended)

Question 13

What is hydroelectric power?

Answer 13

This is produced by allowing water to pass through a turbine by falling through a height.

Question 14

What is wind power?

Answer 14

This is generated when winds are used to directly turn turbines that are used to generate electricity.

Question 15

What is solar power?

Answer 15

This makes use of photovoltaic cells, which produce electricity from light.

Question 16

What is heat?

Answer 16

Heat is the transfer of energy that occurs as a result of a temperature difference.

Question 17

Rate of energy transfer (equation).

Answer 17

The rate of energy transfer is proportional to the temperature difference across the boundary.

ΔT = Tin - Tout

Specified in kelvin.

Question 18

Calculate the rate of energy transfer (power equation)

Answer 18

P = E / t

Where the rate if energy transfer is power, P, amount of energy transfer is E and t is the time in which the energy is transferred.

Question 19

Rate of energy transfer relationships equations (heat conduction equation 2.1)

Answer 19

P = power, A = surface area and l = boundary thickness, T = temperature (K)

P α Α
P α ΔΤ
P α 1/l
P α AΔΤ/l
P = -(kA (ΔT)/l) (2.1)

Where k is a constant of proportionality which changes depending on the material.

Question 20

Calculating the Chang in temperature across multiple layers.

Answer 20

ΔT = -(P/A)(la/ka + lb/kb)

Where P is power, A is surface area and lx and kx are the thickness and thermal conductivity values for each layer.

For more layers you can just add more terms for lx and kx inside the second bracket.

Question 21

Radiation power equation.

Answer 21

P rad = AεσΤ^4

Where ε is the emissivity and σ is the stephan Boltzmann constant. A is area and T is temp in kelvin.

To calculate the net power being emitted its the difference between the power radiated and the radiation received by the radiator.

Question 22

What is rolling resistance?

Answer 22

A type of energy dissipation experienced by wheels rolling over a surface.

Occurs due to the deformation of the wheel as it is squashed and unsquashed by the weight of the car.

Question 23

Equation for the energy dissipated due to rolling

Answer 23

Work (tyres) = F(tyres) x d

Work (tyres) = Mgd

Energy (rr) = C(rr) W(tyres)
= C(rr)Mgd

Where Crr is the coefficient of rolling resistance.

Question 24

Equation for the power dissipated through a type.

Answer 24

P(rr) = E(rr) / t

P(rr) = (C(rr) Mgd) / t

As v = d/t

P(rr) = C(rr) Mgv

Question 25

Air resistance formula

Answer 25

P(air) = (1/2)ρC(D)Av^3

Where ρ is air density, C(D) is the drag coefficient which is unitless, A is the frontal cross sectional area and v is the velocity

Question 26

Comparing air resistance at 2 speeds formula

Answer 26

P(air v2) = (v2/v1)^2 P(air v1)

Question 27

Calculating friction (basic model)

Answer 27

As a general rule of thumb, approximately 15% of the total power is lost to the transmission due to fiction. This is the some as saying 85% of the power from the engine is delivered to the wheels.

Question 28

Proportionality between forces

Answer 28

P(air) α air density

P(air) α speed^3

P(air) α cross sectional area

P(air) α

P(rr) α weight

P(rr) α speed

Question 29

What happens when a car brakes?

Answer 29

The kinetic energy is dissipated mainly as fiction in the braking system.